Current-limiting fuses with balanced low-current and high current interrupting performance



y 5, 1953 P. c. JA as. JR 833,891

CURRENTLIMITING FU ES TH BALANCED LOW-CU NT AND HIGH CURRENTINTERRUPTING PERFORMANCE Filed Oct. 1, 1956 Inventor:

Philip C- Uocobs.dr.

Atty.

rates Patent 0 CURRENT-LIMITING FUSES WITH BALANCED LOW-CURRENT AND HIGHCURRENT INTER- RUPTING PERFORMANCE This invention relates tocurrent-limiting fuses, i. e. to fuses tending to limit the peaks oflet-through currents well below the peaks of available fault currentsThe invention relates particularly to current-limiting fuses forelevated circuit voltages, i. e. circuit voltages of at least severalkilovolts.

To achieve satisfactory current-limitation, i. e. to limit the peaks oflet-through currents to relativelylow values, a number of conditionsmust be met. The casing of the fuse must be filled with a suitablepulverulent arcquenching filler, preferably a silicious filler such asquartz sand. For a wide range'of current carrying capacity andinterrupting capacity ratings the application of multiperforatedribbon-type fuse links is indicated. The fuse links ought to consist ofa metal having a relatively high conductivity and a relatively lowfusing energy. Silver meets these requirements best, yet in someinstances copper ribbons may also be used as fuse links. When these andother conditions required for best current-limiting performance at majorfaults are fully met,'currentlimiting fuses tend to perform ratherpoorly on blowing at relatively small protracted overloads.

it is, therefore, one object of this invention to providecurrentdimiting fuses performing equally well in the entire range fromthe smallest protracted overload which must be interrupted to thehighest available short circuit current, or the steepest rate of rise offault current.

In the past many fuse structures have been evolved intended to achieve areasonably rapid interruption of a circuit both in the range ofrelatively small excess currents and in the range of relatively highexcess currents. This object could not be achieved satisfactorily exceptby resorting to two separate fusible interrupting devices of which oneis particularly adapted to operate in the range of relatively smallexcess currents, and the other particularly adapted to operate in therange of relatively high excess currents.

It is, therefore, another object of this invention to providecurrent-limiting fuses having but one single interrupting means whicheffects sufficiently rapid interruption of electric circuits both in therange of relatively small excess currents and in the range of relativelylarge excess currents. b

Other objects of the invention are to provide currentlimiting fusesprecluding voltage surges on blowing at O highfault currents and moreparticularly to provide current-limiting high-voltagefuses whoseribbon-type fuse links comprise a single means for achieving relativelyrapid interruptions of relatively small currents and relatively slowinterruption of relatively large currents. 65

The foregoing and other general and special objects of the invention andadvantages thereof will appear more clearly from the ensuing particulardescription of the invention, as illustrated in the accompanyingdrawings, wherein: v 70 Fig. 1 is a side elevation of a portion of afuse linkembodying the present invention;

Fig. 2 is a side elevation of a modification of the linkstructure shownin Fig. 1; b

Fig. 3 is a longitudinal section of a current-limiting fuse comprising afuse link as shown in Figs. 1 and 2;

Fig. 4 is a section along 4-4 of Fig. 3,; I I

Fig. 5 is a side elevation of a portion of a simplified version of thestructure shown in Fig. 2; and

Fig. 6 is a side elevation of another simplified version of thestructure shown in Fig. 2. 7

Referring now to Fig. 1 of the drawing, reference character a has beenapplied to indicate a ribbon of a metal having a relatively highconductivity and a relatively low fusing energy. Ribbon a is preferablymade of silver, though it might be made of copper for some applications.Ribbon-type fuse link a is provided with a plurality of necks, i. e.points of reduced cross-sectional area where there is a tendency towardrelatively high current-density, toward heat generation at a relativelyrapid rate, and toward initial fusion and arc initiation. Referencenumerals b and 0 have been applied to indicate punched out portions, orperforations. Perforations b have a substantially rectangular outline,and perforations c are substantially T-shaped. The necks d formedbetween perforations b and c are relatively short and their ohmicresistance is relatively small. Reference letter I has been applied toindicate the relatively short length of necks d. T-shaped perforations cand the edges 2 of link'a jointly define relatively long necks having arelatively high ohmic resistance. Reference letter L has been applied toindicate the relatively long length of necks f. The width w of necks dis small in comparison to .the total width W of link a, e. g. the ratioof w/ W may be in the order of 1:20. I

Each neck f is provided with three serially related restrictedcross-section portions, or subnecks s. When any excess current causesfusion of .neck 1, sub-necks s tend to form series breaks.

Referring now to Fig. 2, numeral 01' indicates a portion of aribbon-type fuse link, preferably made of silver. The outline of thelink is substantialy. rectangular, the link being bounded laterally by apair of parallel edges 2. The middle or center region of link a isprovided with a square perforation b. and a pair of T-shapedperforations 0'. Short necks d having a relatively small ohmicresistance are formed. between perforations b and c. Long necks f havinga relatively high ohmic resistance are formed between the edges e andperforations c. Reference letter I has been applied to indicate thelength of necks d, and reference letter L has been applied to indicatethe length of necks f. The region of link a immediately adjacent necks dand j is covered by an overlay 0 made of a metal having a fusing pointlower than silver, capable of forming with silver alloys having a higherresistivity than silver. Overlay 0 consists preferably of tin, orindium, or suitable alloys of tin or indium. Link a is also providedwith rectangular perforations m forming necks n whose length has beenindicated by reference character I. The length of necks .d' andthelength of necks n is equal and the total width of cross-sectional areaof the two necks d is substantially equal to the. total width orcross-sectional area of each group of three necks n. As a result of thisgeometrical configuration, fusion will occur substantiallysimultaneously atnecks d and n when the rate of rise of current is sorapid as to permit heat exchange phe nomena to be neglected. Each neckis provided with three serially related cross-section portions, orsub-necks s, tending to form series breaks upon blowing of the fuse.

Referring now to'Figs. 3 and 4, reference numeral a" has been applied toindicate a ribbon-typefuse link identicaltto fuse links a .and.a,illustrated in Figs. 1 and 2,

the latter figures being drawn on a considerably larger scale. Fuse linka" is arranged in a tubular casing t" which may consist of glass, aceramic material, or a synthetic-resin-glass-cloth laminate. The ends ofcasing t" are closed by a pair of suitable terminal elements such as apair of metal caps p. Casing z" is filled with a pulverulent siliciousarc-quenching filler r", preferably chemically pure quartz'sand. Metalwashers U clamped by caps p" against casing t" close the ends of thelatter and are each provided with a rectangular perforation y" for thepassage of link a". The ends of link a are firmly clamped in positionbetween casing t" and caps p". Reference characters z have been appliedto indicate two points where the ends of links a" are turned outwardly,out of the narrow gaps formed between casing t" and caps p", andspot-welded to caps p".

The operation of the structures shown in Figs. 1-4 will now bedescribed. Since the ohmic resistance of necks d is considerably lessthan the ohmic resistance of necks f, the current flowing in link a willbe concentrated in necks d. In spite of the high current-densitiesoccurring in necks d, the current carrying ability (or floating current,or minimum fusing current) of link a will be relatively high on accountof the short length of necks d tending to minimize i -r losses, and onaccount of the relatively great width W of link a tending to rapidlydissipate whatever heat is being generated at necks d. On occurrence ofprotracted overloads initial fusion will occur at necks d where thecurrent density in link a is highest. The fact that the resistance ofshunt necks f is relatively high results in a significant voltage dropat both points of initial break, resulting in turn in the formation ofan arc gap of some length. As arc voltage is being built up at thepoints of initial arc formation, the current is being shifted away fromthe regions of necks d and flows through the high resistance necks f.This permits cooling and de-ionization of the initially formed are gaps,and fusion of the relatively long necks 7. Fusion will be initiated atthe three points s where the width, or crosssectional area, of necks fis fairly small and because series breaks are being formed along necksf, the arc voltage formed in the region of necks will be appreciable,tending to drastically reduce the arcing time and the are energy.

The process of interruption at the necks d, f situated in the center oflink a may precede arc initiation at other necks d, j which are situatedaxially outwardly. As a general rule, the long breaks formed at theregions of necks f in the center of link a are sufiicient to interruptrelatively small protracted overload currents. may not be necessary toprovide additional overload current interrupting breaks on the fuselink.

Fig. 2 shows a fuse link wherein overload interrupting necks in additionto the necks a" and f in the center region of the fuse link have beenomitted. In that figure the overlay causes initial fusion to occur atnecks d and f at relatively low temperatures. Final interruption ofrelatively low currents is effected by progressive backburning of necksf. Back-burning is accelerated by virtue of sub-necks s. Necks n' arenot caused to fuse at overloads; these necks form breaks on occurrenceof major fault currents only.

In case of major fault currents necks d and f fuse sequentially as incase of small protracted overloads. In other words, the feature ofsequential fusion of necks d and f is common to both low current andhigh current interruptions. Sequential fusion at two parallel neckregions tends to limit the rate of change of current cident tointerruption of major fault currents.

As indicated above, in the structure of Fig. 2 major Hence it faultcurrents are being interrupted at the serially formed breaks n, d, f, n.

The fuse links shown in Figs. 1 and 2 are preferred embodiments of theinvention. The fuse links shown in Figs. 5 and 6 are based upon exactlythe same principles as those shown in Figs. 1 and 2, yet are not asdesirable in some instances as the latter because of a less favorableratio of current carrying capacity to arc-quenching capacity expressedin terms of area of interaction between link and quenching medium.

Referring now to Figs. 5 and 6, reference characters c" and c have beenapplied to indicate substantially T-shaped perforations in the silverribbons a" and a', respectively.

The structure shown in Fig. 5 comprises but one single T-shapedperforation. One portion of T-shaped perforation c" extendssubstantially parallel to the edges e of link a" defining a relativelylong neck 7" having a relatively high ohmic resistance. The otherportion of perforation 0 extends transversely across link a defining arelatively short neck d having a relatively low ohmic resistance. Necksd and f" fuse sequentially both on protracted overloads and on majorfaults.

In the structure of Fig. 6 the ribbon-type fuse link a' is provided withtwo T-shaped perforations c' of different sizes. The upper perforation chas a portion extending parallel to ribbon edges e which portion islonger than the portion of the lower T-shaped perforation 0' extendingparallel to ribbon edges e. As a result, the upper neck 1 is longer andhas a higher ohmic resistance than the lower neck f'. Both necks f shuntthe very short neck d' which has a very low ohmic resistance. Onoccurrence of protracted overloads as well as on occurrence of majorfaults the interruption of the circuit is initiated by fusion of neckd'. Thereafter the lower neck f fuses and finally the upper neck ffuses.

The term neck has been used in the foregoing in its customary orconventional meaning with reference to a relatively short portion ofrestricted cross-sectional area which defines the point or points ofinitial fusion and are initiation along the active or fusible length ofthe link. In accordance with this object of a neck the length thereof isa relatively small fraction of the active or fusible length of a link.

It will be understood that the sequence in which the three necks d' andf' fuse at major fault currents is much more rapid than the sequence offusion at relatively low currents; yet the sequence fusion feature issufficiently efiective at high currents to limit the rate of change ofcurrent to values tending to preclude damage by voltage surges to theinsulation of the system.

It will be understood that by illustrating herein several forms of fuselinks, I do not intend to limit my invention thereto. It will be furtherunderstood that the invention may take forms other than the structuresdescribed and illustrated herein. It will be apparent to those skilledin the art that various changes and modifications may be made withoutdeparting from the spirit of the invention or from the scope of theappended claims.

It is claimed and desired to secure by Letters Patent:

1. A current-limiting fuse comprising a tubular casing of insulatingmaterial, a pulverulent arc-quenching filler inside said casing, a pairof terminal elements closing the ends of said casing, a ribbon-type fuselink of a relatively high conductivity relatively low fusing energymetal immersed in said filler conductively interconnecting said pair ofterminal elements, said link comprising a relatively short neck portionand said link further compris ing a relatively long neck portion havingan ohmic resistance considerably higher than said relatively short neckportion, and said relatively long neck portion being adapted to shuntsaid relatively short neck portion.

2. A current-limiting fuse comprising a tubular casing of insulatingmaterial, a pulverulent silicious arc-quenching filler inside saidcasing, a pair of terminal elements closing the ends of said casing, aribbon-type fuse link of silver conductively interconnecting said pairof terminal elements, said link comprising a pair of relatively longneck means forming edges of said link and said link further comprisingrelatively short neck means having a substantially lower ohmicresistance than said relatively long neck means situated in the spacebetween said relatively long neck means and shunted by said relativelylong neck means.

3. A current-limiting fuse comprising a tubular casing of insulatingmaterial, a pulverulent arc-quenching filler inside said casing, a pairof terminal elements closing the ends of said casing, a ribbon-type fuselink having a substantially rectangular outline made of a relativelyhigh conductivity relatively low fusing energy metal immersed in saidfiller conductively interconnecting said pair of terminal elements, saidlink being provided with at least one substantially T-shapedperforation, one portion of said perforation extending substantiallyparallel to the edges of said link and defining a relatively long neckhaving a relatively high ohmic resistance and the other portion of saidperforation extending substantially transversely across said link anddefining a relatively short neck having a relatively low ohmicresistance.

4. A current-limiting fuse comprising a tubular casing of insulatingmaterial, a pulverulent silicious arc-quenching filler inside saidcasing, a pair of terminal elements closing the ends of said casing, aribbon-type fuse link of silver immersed in said filler conductivelyinterconnecting said pair of terminal elements, said link being providedwith a pair of substantially T-shaped perforations, each of said pair ofperforations having a portion extending substantially parallel to theedges of said link and defining a relatively long neck having arelatively high ohmic resistance, and each of said pair of perforationshaving a portion extending transversely across said link at right anglesto the edges thereof forming therebetween a relatively short neck havinga relatively low ohmic resistance.

5. A current-limiting fuse comprising a tubular casing of insulatingmaterial, a pulverulent arc-quenching filler inside said casing, a pairof terminal elements closing the ends of said casing, a ribbon-type fuselink of a relatively high conductivity relatively low fusing energymetal immersed in said filler conductively interconnecting said pair ofterminal elements, said link being shaped to provide a pair of parallelcurrent path, one of said pair of current path being relatively shortand having a relatively low ohmic resistance and the other of said pairof current path being relatively long and having a relatively high ohmicresistance, said relatively long current path having a plurality ofserially related points of reduced cross-sectional area tending to formseries breaks upon fusion of said relatively long current path.

6. A current-limiting fuse comprising a tubular casing of insulatingmaterial, a quartz sand filler inside said casing, a pair of terminalelements closing the ends of said casing, a ribbon-type fuse link ofsilver conductively interconnecting said pair of terminal elements, saidlink comprising a relatively short neck portion and said link furthercomprising a relatively long neck portion having an ohmic resistanceconsiderably higher than said relatively short neck portion, saidrelatively long neck portion being adapted to shunt said relativelyshort neck portion, an overlay on said link comprising a metal having afusing point lower than silver capable of forming with silver alloyshaving a higher resistivity than silver, and said overlay being arrangedon said link immediately adjacent to said relatively short neck portionand immediately adjacent to said relatively long neck portion thereof.

7. A fuse link for current-limiting fuses comprising a silver ribbonhaving a pair of substantially T-shaped perforations, each of said pairof perforations having a portion extending substantially parallel to theedges of said link and defining a relatively long neck portion having arelatively high ohmic resistance, and each of said per forations havinga portion extending transversely across said link forming therebetween arelatively short neck portion having a relatively low ohmic resistance.

8. A current-limiting fuse comprising a tubular casing of insulatingmaterial, a pulverulent arc-quenching filler inside said casing, a pairof terminal elements closing the ends of said casing, a ribbon-type fuselink of a relatively high conductivity relatively low fusing energymetal having a predetermined active length immersed in said fillerand-conductively interconnecting said pair of terminal elements, saidlink defining a plurality of current paths of different length, and thelength of each of said plurality of current paths being but a relativelysmall fraction of said predetermined active length.

References Cited in the file of this patent UNITED STATES PATENTS1,631,669 Brown June 7, 1927 2,703,352 Kozacka Mar. 1, 1955 2,720,567Detch Oct. 11, 1955

